Reducing the prevalence of obesity in the population is one of the Healthy People 2010 goals, and increased vigilance is required in all age groups in preventing and reversing overweight and obesity. On an individual level, behavioral factors that influence body weight include eating choices leading to excess calorie intake and physical activity patterns leading to diminished calorie expenditure. A balance between the two over time leads to increased weight. On a population basis, the prevalence of overweight is associated with a myriad of influences, including social, behavioral, cultural, and environmental factors as well as genetic and physiological factors. For a majority of overweight individuals, restoring a balance between energy intake and expenditure is difficult, and therefore there is an increasing emphasis on preventing obesity on a population level. The long-term goal of this research is to prevent further increase in rates of obesity in the population. In the adult population of working age, a majority of the day is spent in the worksite, suggesting that interventions at the worksite level may offer the opportunity for success in this age group. We propose to develop and test a comprehensive intervention with simple messages that will integrate changes in dietary intake with changes in energy expenditure, while simultaneously modifying structural and environmental factors to promote social support and opportunities for behavioral change. We will recruit and randomize 48 worksites to a two year intervention in which we will: build a physical activity intervention combining increased daily physical activity and regular, structured exercise; build a dietary intervention that will promote lower calorie intake; increase worksite access to both healthy foods and physical activity. Our primary aim is to evaluate the effectiveness of the intervention in reducing or maintaining body mass index in a randomized controlled trial of worksites. We will compare changes in body mass index in intervention versus control worksites using two cross-sectional surveys at baseline and follow-up. Additional assessments using biomarkers and fitness measures will be performed in a subset of employees. Our team has considerable experience with interventions at the worksite level and substantial expertise in obesity prevention. We anticipate that this project will yield important contributions to the understanding of obesity prevention.